Epoxy resin compositions, on curing, produce cured products with superior mechanical characteristics, electrical characteristics and adhesion, and are consequently widely used for applications such as sealants for electronic materials, paints, paving materials, and adhesives. Furthermore, recently epoxy resin compositions have also started to be used as matrix resins for fiber reinforced composite materials, with wide ranging applications, from aircraft components through to everyday items such as fishing rods and golf club shafts.
Methods of forming fiber reinforced composite materials with good levels of productivity which are currently attracting interest include resin transfer molding (RTM) methods in which a liquid resin composition is injected into a mold which has been filled with a reinforced fiber preform material, and the mixture is subsequently cured, draw molding methods in which a liquid resin composition is impregnated into a reinforced fiber yarn, and this impregnated yarn is then drawn into a mold and cured in a short time period, and filament winding molding methods in which a liquid resin composition is impregnated into a reinforced fiber yarn, and this impregnated yarn is then wound around a mandrel and cured. Of these methods, the RTM method offers the advantage of enabling cured products of complex shapes to be produced at low cost.
In order to form a fiber reinforced composite material using these types of highly productive methods, the matrix resin must be of low viscosity, the low viscosity must be sustainable, and the cured product produced by curing the resin must display superior mechanical characteristics and heat resistance. Particularly in the case of RTM molding methods, matrix resins in which a low viscosity can be sustained with good stability over long periods are in great demand. In addition, when large items are produced by RTM molding, because it is technically difficult to heat the mold uniformly to the appropriate temperature, low viscosity matrix resins which can be impregnated with reinforced fiber even at room temperature are required. When the atmospheric temperature is raised, thermosetting resins typically display a decrease in initial viscosity, although the rate with which the viscosity rises increases. In contrast, when the atmospheric temperature is lowered, the rate of viscosity increase slows and the resin viscosity becomes more stable, although the viscosity level itself increases. Consequently, it has proved very difficult to sustain a low viscosity with good stability over long periods at room temperature.
A liquid form matrix system epoxy resin composition with a dynamic viscosity at 40° C. of no more than 400 mPa·s has been disclosed in Japanese Unexamined Patent Application, First Publication No. Sho 57-51719. However, the elastic modulus of the cured product produced by curing this epoxy resin composition is unsatisfactory, and the resin is unsuitable as a matrix resin for a fiber reinforced composite material. Furthermore, if this resin is used within an RTM molding method, then additional problems arise such as the need to heat the mold during impregnation of the reinforced fiber.
Furthermore, low viscosity epoxy resin compositions which use a carboxylic acid anhydride as the curing agent have been widely reported as low viscosity liquid epoxy resins, but such compositions require considerable care in handling as the carboxylic acid anhydride readily absorbs moisture.